Abstract:

A system, device, and method for aggregating traffic in a network, such as
for wireless backhaul traffic in a cellular communication system. UE
device traffic is sent to and from an access bridge, which is one node in
an access network formed into a ring topology, and from the access bridge
to a core bridge in communication with a core network of the
communication system. According to a preferred embodiment, the access
bridge and the core bridge each have a LAG component configured according
to IEEE 802.1D and IEEE 802.3-2005, and a ring component configured
according to IEEE 802.1Q or 802.1AD. In operation, this LAG component
load balances communication traffic coming into the bridge node onto one
of two VLANs formed of portions of the access network ring. In the event
of a fault condition affecting one of the two VLANs, the fault VLAN is
removed from the aggregation scheme until the fault condition is
alleviated.

Claims:

1. A network node for use in an Ethernet ring network, the network node
comprising:a LAG (link aggregation) component configured according to
IEEE 802.1D and IEEE 802.3-2005 and comprising a first port for receiving
communications from non-ring entities, and further comprising a second
port and a third port for forwarding the received communications as
untagged frames; anda ring component configured according to IEEE 802.1Q
or IEEE 802.1AD and comprising a first port and a second port for sending
tagged frames on, respectively, a first VLAN and a second VLAN (virtual
local area network) formed in the Ethernet ring network, and further
comprising a third port and a fourth port for receiving untagged frames
from, respectively, the second port and the third port of the LAG
component.

2. The network node of claim 1, wherein the ring component is arranged so
that untagged frames received at the third port of the ring component are
forwarded as tagged frames on the first VLAN, and wherein the untagged
frames received at the fourth port of the ring component are forwarded as
tagged frames on the second VLAN.

3. The network node of claim 1, wherein the LAG component is arranged to
load balance the untagged frames forwarded to the ring node.

4. The network node of claim 3, wherein the LAG component is further
arranged to, upon receiving a notification that a fault affecting one of
the first VLAN and the second VLAN has occurred, forwards all of the
received communications on the one of the second or third ports that is
not associated with the affected VLAN.

5. The network node of claim 4, wherein the LAG component is further
arranged to, upon receiving a notification that the fault has been
alleviated, resume load balancing the untagged frames forwarded to the
ring node.

6. A method of aggregating traffic to a core network via an access network
comprising a plurality of network nodes, the method comprising:forming a
ring topology comprising the plurality of network nodes;selecting a core
bridge from among the plurality of nodes, the core bridge in
communication with the core network and comprising a ring component and a
LAG component;establishing an access bridge comprising a ring component
and a LAG component;enabling LAG protocol traffic forwarding from the
access bridge;configuring a first VLAN comprising a portion of the
plurality of network nodes and extending from the access bridge to the
core bridge;configuring a second VLAN comprising the remainder of the
plurality of network nodes and extending from the access bridge to the
core bridge; andload balancing traffic from the access bridge to the core
via the first VLAN and the second VLAN.

7. The method of claim 6, wherein the access network is an Ethernet
network.

8. The method of claim 7, further comprising detecting whether STP
(spanning tree protocol) is being used for determining network routing
paths.

9. The method of claim 8, further comprising disabling STP for the nodes
of the access network.

10. The method of claim 6, wherein the LAG component of the access bridge
is configured to be operable according to IEEE 801D.

11. The method of claim 10, wherein the ring component of the access
bridge is configured to be operable according to IEEE 801Q.

12. The method of claim 11, further comprising:forwarding untagged frames
from a first port of the access bridge LAG component to a first port of
the access bridge ring component for aggregation to the core network via
the first VLAN; andforwarding untagged frames from a second port of the
access bridge LAG component to a second port of the access bridge ring
component for aggregation to the core network via the second VLAN.

13. The method of claim 12, fuirther comprising:forwarding tagged frames
from a third port of the access bridge ring component to a neighbor node
on the first VLAN; andforwarding tagged frames from a fourth port of the
access bridge ring component to a neighbor node on the second VLAN.

14. The method of claim 6, further comprising performing a fault detection
procedure.

16. The method of claim 15, further comprising detecting a fault condition
affecting one of the VLANs and transmitting a notification to the access
bridge LAG component.

17. The method of claim 16, further comprising, upon receipt of
notification of a fault condition affecting one of the VLANS, forwarding
untagged frames from the access bridge LAG component only for the
unaffected VLAN.

18. A communication system comprising a plurality of access nodes
configurable in a ring topology, wherein the plurality of access nodes
comprises a first bridge node and at least a second bridge node, each
bridge node comprising a ring component for establishing a first VLAN
comprising a first subset of the plurality of access odes and a second
VLAN comprising a second subset of the plurality of access nodes, and
each bridge node further comprising a LAG component in communication with
the ring component, the LAG component for communicating with an entity
outside of the plurality of access nodes.

19. The communication system of claim 18, wherein the LAG component is
configured according to IEEE 802.1D and IEEE 802.3-2005.

20. The communication system of claim 18, wherein the ring component is
configured according to IEEE 802.1Q or IEEE 802.1AD.

21. The communication system of claim 18, wherein the communication system
further comprises a core network, and wherein the first bridge node is a
core bridge for communicating between the first and second VLANs and the
core network.

22. The communication system of claim 21, wherein the communication system
is a wireless network.

23. The communication system of claim 22, wherein the second bridge node
is an access bridge for receiving communications from UE devices, and
wherein the wherein the LAG component of the access bridge is arranged to
load balance the untagged frames forwarded to the ring node by the ring
component of the access bridge.

Description:

FIELD OF THE INVENTION

[0001]This invention relates generally to the field of wireless
communication systems and, more particularly, to a device, system, and
method providing fault protection and load balancing in aggregation
networks.

BACKGROUND OF THE INVENTION

[0002]Communication networks include a large number of interconnected
components that enable a UE (user equipment) device with network access
to communicate with other such devices located within the network
coverage area, and with devices connected through other networks as well.
The architecture of any communication system in modern use is generally
somewhat hierarchical, that is, widely-disbursed access points allow
users to connect with a more centralized core network, which is able to
route the voice and data information involved in a great many
communication sessions.

[0003]For example, a wireless network includes many access nodes,
typically antennas connected to BTSs (base transceiver stations),
distributed over the network coverage area. A network subscriber using an
appropriate device can establish communication with the network though
one of these access nodes. During a communication session, voice and data
information transmitted to the access node is then relayed to a core
network for routing to its destination. Information destined for the
subscriber is sent to the appropriate access node for transmission to the
UE device.

[0004]A wireless network coverage area is often divided into cells, or
relatively-small geographic areas having (normally one) antenna for
radio-frequency communication with UE devices located within or near the
cell. The advantage of a cellular network is that mobile phones can
transmit at relatively low power to a near-by antenna, which conserves UE
battery power and also allows the reuse of the same frequency channels in
non-adjacent cells separated by only a relatively-small distance.

[0005]For efficiency, a number of access nodes may be grouped together
into an access network, which aggregates the voice and data traffic
associated with many UEs for communication with the core network through
one or a limited number of network nodes. In a wireless network, this
process is sometimes referred to as wireless backhaul. There are several
network architectures that may be employed for constructing the access
network for backhauling.

[0006]Ethernet is one such architecture. An Ethernet network is one that
is configured and operated according to the standard IEEE 802.3 and a
number of related standards. For example, IEEE 802.1Q and IEEE 802.1AD
specify the configuration of virtual bridged local area networks and
provider virtual bridged local area networks, respectively, including
VLAN (virtual local area network) tagging, and IEEE 802.1D specifies the
configuration of MAC (media access control) bridges, including the use of
STP (spanning tree protocol).

[0007]Ethernet networks may be organized or configured into "rings". A
ring configuration is formed by examining a set of Ethernet nodes capable
of routing information traffic, and determining a manner of routing that
provides a pair of redundant paths from an originating node to a
destination node. Note that in this sense, the term "ring" is a general
topological reference, but does not necessarily connote a specific
physical layout. Nor does the term imply that traffic is ordinarily
routed in a loop or circular path. The nature of a ring, as that term is
used here, will become more apparent in light of the exemplary
embodiments described below.

[0008]There are different ways for implementing Ethernet configurations in
the wireless backhaul context. One way, for example, is to provide a
plurality of suitably interconnected bridges running a routine referred
to as STP (spanning tree protocol). While STP is relatively
cost-effective, in the event of a fault, that is a breakdown somewhere in
the configured ring, the recovery procedure it provides may be too slow
to qualify as carrier-grade operation. Another example is a RPR
(resilient packet ring) scheme, which provides better fault protection
than STP but has not proven cost-effective and probably for this reason
has been implemented on only a limited basis.

[0009]Needed, then, is a solution for wireless backhaul aggregation
networks that is not only acceptable from a cost perspective but also
provides adequate load balancing and fault protection in operation.

SUMMARY OF THE INVENTION

[0010]The system, device, and method embodiments of the present invention
provide for efficient aggregation of data in a communication system, for
example in a wireless network employing an Ethernet network as an access
network for backhaul aggregation to a core network.

[0011]In one aspect, the present invention is a network node for use in an
Ethernet ring network, including a LAG component configured according to
IEEE 802.1D and IEEE 802.3-2005 and including a first port for receiving
communications from non-ring entities as well as a second port and a
third port for forwarding the received communications as untagged frames
to a ring component configured according to IEEE 802.1Q or IEEE 802.1AD.
The ring component includes a first port and a second port for sending
tagged frames on, respectively, a first VLAN and a second VLAN formed in
the Ethernet ring network, and also includes a third port and a fourth
port for receiving untagged frames from, respectively, the second port
and the third port of the LAG component.

[0012]In another aspect, the present invention is a method of aggregating
traffic to a core network via an access network comprising a plurality of
network nodes, including the steps of forming a ring topology of the
plurality of network nodes, selecting a core bridge from among the
plurality of nodes in the ring, the core bridge for communicating with
the core network, and establishing an access bridge. Preferably both the
core bridge and the access bridge include a ring component and a LAG
component. The method further includes the steps of enabling LAG protocol
traffic forwarding from the access bridge, configuring a first VLAN and a
second VLAN, each including a portion of the plurality of network nodes
and extending from the access bridge to the core bridge. The traffic is
then load balanced, that is, transmitted in a load-balanced fashion, the
access bridge to the core via the first VLAN and the second VLAN.

[0013]In yet another aspect, the present invention is a communication
system formed of a plurality of access nodes and configurable into a ring
topology, wherein the plurality of access nodes includes at least a first
bridge node and a second bridge node, each bridge node including a ring
component for establishing a first VLAN of a first subset of the
plurality of access odes and a second VLAN of a second subset of the
plurality of access nodes, and each bridge node further including a LAG
component in communication with the ring component, the LAG component for
communicating with an entity outside of the plurality of access nodes.

[0014]In yet another aspect, the present invention is an access network
including a plurality of access nodes, the access network for aggregating
communication-system traffic between a core network and one or more user
devices. The access network according to this aspect of the present
invention includes at least two bridge nodes, each bridge node including
both a ring component and a LAG component.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]The foregoing and other advantages of the invention will become
apparent upon reading the following detailed description and upon
reference to the drawings, in which:

[0016]FIG. 1 is a block diagram of a communication system illustrating an
access network including a plurality of access nodes that may be
configured to aggregate traffic between the access nodes and the core
network;

[0017]FIG. 2 is a block diagram of a communication system illustrating the
access network of FIG. 1 configured to according to an embodiment of the
present invention;

[0018]FIG. 3 is a block diagram illustrating the configuration of a bridge
node according to an embodiment of the present invention;

[0019]FIG. 4 is a block diagram of a communication system illustrating the
occurrence of an exemplary fault condition in the access network of FIG.
1 while configured to according to the embodiment of FIG. 2; and

[0020]FIG. 5 is a flow diagram illustrating a method for aggregating
communication traffic in an access network according to one embodiment of
the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

[0021]The present invention is directed to a manner of using an access
network formed in a ring configuration to aggregate communication
traffic. As one example, the access network may be an Ethernet ring used
for aggregating wireless backhaul traffic to a core network.

[0022]FIG. 1 is a block diagram of a communication system 100 illustrating
an access network 120 including a plurality of access nodes that may be
configured to aggregate traffic between the access nodes and the core
network 190. In this embodiment, communication system 100 is a cellular
telephone system, and access network 120 is configured as the interface
between users of the system and the core network 190 though which a
majority of voice and data traffic may be routed toward its final
destination.

[0023]In the embodiment of FIG. 1, access network 120 includes access
nodes 130, 140, 150, 160, and 170. Note that the present invention
requires no particular access node configuration unless explicitly stated
in claiming a particular embodiment or apparent from the context. These
nodes are formed in a ring formation, meaning that any one node may
communicate with others in the access network via either of two neighbor
nodes accessible though a first or second port, respectively. For
example, node 140 may forward frames destined for any of the other nodes
through first port 143. Node 130 receives frames at its first port 132
and, if the frames are directed to a different node of the access
network, forwards them in the appropriate direction. Frames addressed to
node 160, for example, will be placed on the second port 133 of node 130
and received at first port 173 of node 170. Node 170, in turn, forwards
the frames on port 174 to the second port 163 of node 160, their final
destination. In the other direction, node 140 could also have forwarded
the frames on second port 144 to its neighbor node 150, where they would
be received on first port 153 and forwarded on second port 154 to be
received on a first port 162 of node 160.

[0024]Briefly, nodes 140, 150, and 170 are illustrated in FIG. 1 as access
nodes. For simplicity, each of these nodes will be described herein as
having a first port (143, 153, and 173, respectively) and a second port
(144, 154, and 174, respectively) that receive and forward frames on the
ring itself. Each of them is also illustrated as having two additional
ports; a third port (141, 151, and 171, respectively) and a fourth port
(142, 152, and 172, respectively). The additional ports are for other
(non-ring) communications, such as for communicating with UE (user
equipment) devices via BTSs (base transceiver stations) and so forth. Of
course, each node may well have more (or in some cases fewer) ports than
are illustrated here. And ports may be re-assigned for ring or non-ring
communication as needed.

[0025]Nodes 130 and 160 serve special roles in implementing the present
invention and are illustrated differently for this reason. Node 130 is,
in the embodiment of FIG. 1, a core bridge that communicates between the
ring-topology access network 120 and the core network 190 via, for
example, a port 131. Core bridge 130 is also shown as having a first port
132 and a second port 133 that communicate, respectively, with the first
port 143 of access node 140 and the first port 173 of access node 170.
Node 136 is, in the embodiment of FIG. 1, an access bridge that
communicates between the ring-topology access network 120 and various UEs
via, for example, a port 161, which may be in communication with one or
more BTSs or similar components. Access bridge 160 is also shown as
having a first port 162 and a second port 163 that communicate,
respectively, with the second port 154 of access node 150 and the second
port 174 of access node 170. Again, core bridge 130 and access bridge 160
may have additional ports for other functions, and ports may be
re-assigned from one function to another. In addition, it is noted here
that any properly-configured node of access network 120 may serve as an
access bridge or a core bridge according to the present invention. It is
preferable, in fact, that each access node be able to function as an
access node for transmitting user traffic to and from the core network.

[0026]FIG. 2 is a block diagram of communication system 100 illustrating
the access network 120 of FIG. 1 configured to according to an embodiment
of the present invention. Specifically, FIG. 2 illustrates (using broken
lines) the configuration of two VLANs, namely, VLAN 110 and VLAN 111.
VLAN 110 extends from the second port 163 of access bridge 160 to the
second port 133 of core bridge 130, and includes node 170. VLAN 111
extends from the first port 162 of access bridge 160 to the first port
132 of core bridge 130, and includes nodes 140 and 150. Note that while
only a relative few nodes make up the two VLANs, in most actual
implementations there are expected to be a much larger number VLAN 110
and VLAN 111 are used to aggregate traffic between access bridge 160 and
the core network via core bridge 190 in a load balanced manner. The
configuration of an access bridge or core bridge according to the present
invention will now be described in greater detail.

[0027]FIG. 3 is a block diagram illustrating the configuration of a bridge
node 260 according to an embodiment of the present invention. Bridge node
260 may be, in this embodiment, either an access bridge or a core bridge.
Although there may be differences between the two nodes in certain
implementations, they both include the same or analogous essential
features in accordance with the present invention. Specifically, bridge
node 260 includes a ring component 264 having a first port 262 and a
second port 276 that, in implementation, will send and receive frames
via, respectively, a first and second VLAN. These frames will be
appropriately tagged for delivery to their proper destination. In a
preferred embodiment, the ring component is configured to operate in
compliance with specification IEEE 802.1Q or IEEE 802.1AD.

[0028]Bridge node 260 also includes a LAG (link aggregation) component
272, so-called herein because it is configured in compliance with IEEE
802.1D and 802.3-2005. Lag component 272, as shown in FIG. 3, includes a
port 274 for non-ring communications. In the case of an access node, this
may include communications with UE devices such as mobile telephones over
a wireless interface, for example, via an antenna connected to a BTS.
Note that while other components may be required or desirable for such
communications, their exact number and configuration is not important to
describing the present invention. If bridge node 260 is assigned as a
core bridge in accordance with the present invention, port 274 will be
for communication with a component of the core network.

[0029]Lag component 272 and ring component 264 must of course be able to
communicate with each other. Note that they may or may not, however, be
co-located within the same physical component. In case the two components
are not collocated the required forwarding of LACP (link aggregation
control protocol) control frames is ensured by 802.1ab-REV and MEF 10
(Metro Ethernet Forum 10). In the embodiment of FIG. 3, LAG component 272
includes a port 270 and a port 280, which communicate, respectively, with
ring component port 268 and ring component port 278. In operation, the
LAG component 272 load balances incoming (for example, via port 274)
traffic by forwarding it toward its destination on either one of a first
VLAN, originating at ring component first port 262, or a second VLAN
originating at ring component second port 276. Ring component port 268 is
configured with a PVID (port VLAN identifier) identified with the first
VLAN and ring component port 278 configured with a PVID associated with
the second VLAN. As untagged frames arrive at ports 268 and 278, they are
tagged for sending on the first and second VLANs, respectively. The
tagged frames are sent on ports 262 and 276.

[0030]By aggregating traffic in this manner, for example, using the access
network 120 via VLAN 110 and 111 (shown in FIGS. 1 and 2), load balancing
is achievable for the ring configuration. The present invention also
facilitates efficient fault protection as well, as described with
reference to FIG. 4.

[0031]FIG. 4 is a block diagram of a communication system 100 illustrating
the occurrence of an exemplary fault condition in the access network 120
of FIG. 1 while configured to according to the embodiment of FIG. 2. In
the example of FIG. 4, a fault has occurred between nodes 150 and 160.
This could, of course, involve a problem with either or both of those
nodes or with the transmission medium between them.

[0032]The fault may be detected, for example, by the absence of CCMs
periodically sent for this purpose by MEPs (maintenance end points; not
separately shown) on the LAG component. In either case, traffic from
access bridge 160 can no longer be successfully sent to the core bride
via (former) VLAN 111 (the "fault VLAN"), which now extends only from
port 153 of node 150 to port 132 of node 130. Former VLAN 111 is
therefore not a useful part of the aggregation scheme.

[0033]When the relevant LAG component of either the access bridge 160 or
the core bridge 130 receives notification that a VLAN is not available do
to a fault, it removes the port on the LAG component corresponding to the
fault VLAN from the current aggregation scheme and instead forwards
traffic on the port on the LAG component corresponding to the operational
VLAN. As should be apparent, the VLAN 110 is, in this scenario, already
configured, and no further convergence or reconfiguration is required. In
many cases, this allows for faster recovery than with existing schemes,
such as those employing STP.

[0034]FIG. 5 is a flow diagram illustrating a method 300 for aggregating
communication traffic in an access network according to one embodiment of
the present invention. At START it is assumed that the components
necessary to performing the method 300 are both available and
operational. The process then begins with forming a plurality of nodes
into a ring topology (step 305). In a ring topology, each network node in
the ring has two neighbor nodes with which it may communicate through
identified ports. When the nodes of the ring are operational, each node
in the ring may communicate with any other ring node via either of its
two neighbor nodes. Note that the network nodes forming the ring topology
may be physically connected into a ring, or may simply configure
available ports to form a functioning ring. Although formed into a ring
in this manner, each node may use other available ports for different
connections as well.

[0035]Once a ring topology has been formed, a core bridge is selected
(step 310). The core bridge is a ring node that includes both a ring
component and a LAG component, as described in more detail with reference
to FIG. 3. The core bridge must, as its name implies, communicate with
the core network. In this embodiment, a virtual port is configured
through which traffic from the ring is passed to the core network. Note
that while in this embodiment one network node is selected as the core
bridge, others may be functionally capable of doing so.

[0036]An access bridge may then be established (step 315). The access
bridge is a ring node that also includes a ring component and a lag
component, as described above. In this embodiment, communication traffic
to and from UE devices is collected by one or more BTSs associated with
the established access bridge. The access bridge will use the ring to
aggregate this traffic to and from the core network.

[0037]In order to do this, according to this embodiment of the present
invention, once the access bridge is established, LAG (link aggregation)
protocol is enabled (step 320) for the ring. If necessary (as indicated
by broken lines in FIG. 5), STP (spanning tree protocol) is disabled
(step 325), as STP convergence is incompatible with this embodiment of
the invention. In some embodiments (not shown), the method also includes
detecting whether in fact STP is in use when the ring topology is formed
at step 305.

[0038]According the embodiment of FIG.5, a first VLAN is then configured
(step 330). The first VLAN extends from a first port on the ring
component of the access bridge to a first port on the ring component of
the core bridge. A second VLAN is also configured (step 335), the second
VLAN extending from a second port on the ring component of the access
bridge to a second port on the ring component of the core bridge. As can
be seen in FIG. 3, where VLANs 110 and 111 are illustrated, each node of
the formed ring is allocated to one or the other VLAN, but not both. As
noted above, however, each of the network nodes may join other VLANs or
be connected in other ways through other ports.

[0039]Traffic from the access bridge to the core is then load balanced
(step 340). That is, traffic from the access bridge is forwarded in a
load-balanced fashion using LAG protocols. Traffic from the core network
destined for the access bridge is load balanced on the first and second
VLANs in a similar manner. This load balance routing is continued until
the network is reconfigured (not shown) or until a fault is detected.

[0040]In this embodiment of the present invention, fault protection is
also addressed. The ring network is checked for connectivity by
transmitting CCMs (connectivity check messages) (step 345). An MEP in
each LAG component port sends and receives CCMs toward its respective
remote LAG component port via its respective VLAN. If no CCM is received
as expected, a fault is presumed. Of course various factors may affect
the delivery of CCMs, and a fault may in some embodiments not be presumed
with the first missed CCM, but rather after a predetermined number of
expected CCMs do not arrive within a given time period.

[0041]If a fault is detected (step 350), then the port corresponding to
the faulty VLAN on the LAG component is removed from the aggregation
scheme (step 355). This may, for example, involved the sending of a fault
alarm to one or more other nodes, and preferably to the network operator
as well (step not shown) so that appropriate action may be taken to
alleviate the fault condition. In practice, of course, this involves
simply reconfiguring the LAG component so that no traffic is provided to
the VLAN that may no longer be used. All traffic from the access bridge
to the core is then routed via the remaining (that is, operational) VLAN
(step 360). For example, referring to FIG. 4, when a fault occurs between
access bridge 160 and node 150, VLAN 111 cannot be used. In this example,
at step 355 access bridge 160 would then continue direct all traffic to
the core network 190 via VLAN 110, at least until the fault condition can
be remedied.

[0042]In this regard, it is noted that a detected fault may or may not
represent a real fault. That, is a false alarm may result from a failure
to receive CCMs due to other reasons. Since there is no way of
immediately determining if an alarm represents a real fault, of course,
the port corresponding to the VLAN perceived to be faulty is removed from
the aggregation scheme nevertheless. Optionally (as represented by the
broken line in FIG. 5), the process may return to step 345 and send out
CCMs as if both VLANs were operational. If fault condition is remedied,
or if a perceived fault turns out not to be an actual fault, then the LAG
components adjust and return to transmitting load balanced traffic on
both VLANs at step 340.

[0043]Note that the method described above is intended to illustrate one
embodiment of the present invention, but other embodiments consistent
with the spirit of the invention may be implemented as well. For example,
the sequence of steps described above may be performed in any
logically-consistent order unless an embodiment specifies a particular
sequence. Some steps, such as configuration of the first VLAN and the
second VLAN, maybe performed simultaneously instead of sequentially. In
addition, in some embodiments, the process of the present invention my
include steps additional to those described above, in others, some steps
may be deemed optional or not performed at all.

[0044]In this manner, the present invention provides a way of aggregating
communication traffic, for example in a wireless backhaul environment
implementing an Ethernet-ring access network. The present invention
provides for the efficient use of network resources through load
balancing, as well as enhanced fault protection. In most implementations,
the solution of the present invention is both scalable and cost
effective.

[0045]The present invention may be embodied in other specific forms
without departing from its spirit or essential characteristics. The
described embodiments are to be considered in all respects only as
illustrative and not restrictive. The scope of the invention is,
therefore, indicated by the appended claims rather than by the foregoing
description. All changes that come within the meaning and range of
equivalency of the claims are to be embraced within their scope.